This invention relates to solar receivers and in particular solar receivers for the collection of concentrated solar radiation.
Solar receivers of the kind to which the invention relates are typically used in conjunction with solar focusing devices such as parabolic reflectors. They convert the optical energy of the incident radiation into heat commonly for the production of electricity via gas and/or steam turbines or to drive chemical reactors.
The efficiency of solar receivers and the reactors which they supply is reduced in at least three ways. Some of the radiation which falls on the receiver is reflected out of the receiver. Some energy is lost by conduction of heat away from the receiver to the surrounding atmosphere through the walls of the receiver and some energy is wasted through radiative losses.
Reflective losses can be minimized by coating the radiated surface with a low reflective substance and shaping the receiver to cause the radiation to be reflected many times before leaving the receiver while conductive losses can be minimized by insulating the receiver.
The losses which are the most difficult to minimise are radiative losses. These are caused by the hot surfaces of the receiver, usually being the radiated surfaces, radiating energy away from the receiver in the form of electromagnetic radiation. The energy radiated away and the wavelengths at which radiative losses occur, both increase with temperature. Radiative losses are difficult to control because the solar radiation has to enter the receiver to heat the working fluid, and if radiation can enter the receiver it can also exit through the same opening.
Known solar receivers currently employ two methods to reduce radiative losses. One such known receiver uses a selective coating placed on the radiated portion of the receiver. This allows solar radiation below a certain wavelength to be absorbed by the receiver and reflect radiation having longer wavelengths. The reflected wavelengths correspond to those which result in the greatest radiative losses. This method is normally used at lower temperatures of approximately 400xc2x0 C. At higher temperatures, the wavelength of the radiated radiation is shorter and increasingly overlaps with that of the incident solar radiation. Thus, using a selective coating to reduce the radiative losses at higher temperatures results in a larger proportion of the solar radiation being reflected, which reduces the theoretical maximum efficiency of the receiver.
A second method employed by other known receivers which operate at higher temperatures is to design the receivers to take advantage of receiver partitioning. In such an arrangement it is typical to use a receiver which is circular in horizontal section. The rim of the receiver having a number of circular receivers each comprising a coiled spiral tube cavity similar to that shown in U.S. Pat. No 4,449,514. The inner portion of the receiver consists of one receiver similar to that disclosed in EP 552732.
The outer receivers around the rim are heated to a lower temperature by solar radiation on the edge of the focus of the incident radiation which has a relatively low concentration ratio. The inner high temperature portion of the receiver heats the pre-heated working fluid from the centre of the focus of the incident radiation to a temperature of approximately 1300xc2x0 C. Thus only the inner portion of the receiver becomes hot enough to produce significant radiative losses. However, with insolation levels at 80% of maximum values acceptable, and when heating working fluids to about 1000xc2x0 C., this system still loses approximatelly 25% of incoming solar radiation via radiative losses.
By controlling the flow rate of the working fluid, receivers usually operate with the exit temperature of the working fluid being approximately constant irrespective of insolation level. Thus the temperature inside the receiver is also constant and therefore, radiative losses are approximately constant when the receiver is in use at any operating insolation level. This means that the energy lost via radiative losses at lower insolation levels is a larger proportion of the incoming radiation. When as much energy is radiated away from the receiver as falls on the receiver, the efficiency approaches zero.
These losses can be partly reduced by using high concentration ratios of solar radiation which allow use of a smaller receiver for a given power output level. However, there is an upper limit to the feasible concentration ratios, and therefore, the maximum operating temperatures at lower insolation levels.
The present invention improves receiver efficiency by reducing radiative losses at all insolation levels.
According to the present invention a solar energy receiver assembly comprises: a receiver body having at least two regions formed therein for the acceptance of solar radiation; means for splitting radiation incident upon the receiver assembly into at least two wavelength bands so that a different band of radiation is incident upon each region; and a means associated with at least one of the regions for substantially preventing wavelengths of electromagnetic radiation longer than those incident upon the region from leaving the region.
Preferably each of the regions has an associated means for substantially preventing wavelengths of electromagnetic radiation longer than those incident upon the region from leaving the region.
Preferably the or each means for substantially preventing wavelengths longer than those incident upon the region from leaving the region comprise a filter. The filter may be linear, concave or convex.
It will be appreciated that there may exist a space between the surface on which the radiation is incident and the means for preventing radiation. This may be advantageous as it reduces the concentration of radiation passing through the means for preventing radiation.
While there are several means of dispersing solar radiation in order that different wavelength bands fall on each receiver, the preferred means, described in detail below, comprise passing the radiation through a prism and/or reflecting it from a reflection grating.
The dispersion of the radiation may be carried out at the receiver assembly itself or by a primary collection means which may be a parabolic trough, dish or heliostat field.
In a preferred example, a separate receiver or group of receivers is associated with each of the regions. Alternatively, a single receiver comprising a number of receiving sections may be provided such that different wavelength bands of radiation are incident on different sections of the receiver.
It is preferred that the receiver assembly comprises at least two receivers. These receivers are designed to operate at different temperatures. In a receiver assembly having two receivers, longer wavelength radiation is directed to be incident on a low temperature receiver and shorter wavelength radiation directed to be incident on a high temperature receiver. The low temperature receiver emits radiation which is of longer wavelength than the radiation incident thereon. The greater portion of this emitted radiation is preferably of longer wavelength than the majority of the long wavelength solar radiation which is incident upon the receiver. Therefore, in preferred embodiments, a filter is placed over the opening in the low temperature receiver. This filter allows incident radiation to pass through it but does not allow the passage of radiation of longer wavelength emitted from the receiver. Thus, the radiation emitted by the receiver is retained within the receiver assembly due to the absorption or reflection of this radiation by the filter.
The high temperature receiver of such an embodiment operates in a similar way. Short wavelength solar radiation is directed to be incident upon the high temperature receiver. The higher temperature of this receiver gives rise to shorter wavelength radiation emissions than the low temperature receiver. However these emissions are of a longer wavelength than the incident radiation. Therefore, in a similar manner to the low temperature receiver, the use of a filter in conjunction with the high temperature receiver allows passage of short wavelength solar radiation but not the longer wavelength emitted radiation. Such a filter used on the high temperature receiver preferably does not transmit wavelengths which are as long as those transmitted by the low temperature receiver. Optionally, only one or not all of the receivers have an associated filter.